1. Field of the Invention
The present invention relates to a technique for extracting ridge directions in respective zones from a fingerprint image divided into a plurality of zones. In particular, the present invention relates to a ridge direction extraction device and the like, capable of extracting ridge directions accurately even in areas such as near core and delta where ridge directions are unstable.
2. Description of Related Art
Conventionally, various proposals have been made for ridge direction extraction devices.
For example, conventional art shown in Japanese Patent Laid-Open Publication No. 52-97298 or in its U.S. Pat. No. 5,717,786 (Patent Document 1) utilizes a fact that relating to the direction of a streak in a pixel of a streaked pattern image, contrast of gray scale is smaller in a direction same as the streak but is larger in a direction orthogonal to the streak, whereby extreme values in the amount of gray scale contrast with reference to predetermined multiple quantizing directions are calculated to thereby determine the direction of streak from the extreme values.
In Japanese Patent Laid-Open Publication No. 8-7097 (Patent Document 2), a gradient vector is calculated in a pixel in which ridge direction is to be defined to thereby determine the ridge direction of each pixel.
Further, in Japanese Patent Laid-Open Publication No. 2002-288641 or in its U.S. Pat. No. 7,027,626 (Patent Document 3), two-dimensional Fourier transformation is performed in a local area in which ridge directions are to be defined, and by analyzing the peak of the resultant Fourier-transformed plane, ridge directions in the local area are determined.
For reducing the calculation amount, ridge directions are often extracted by a small area (zone) unit of about 8*8 pixels without being calculated for all pixels.
In the conventional art described above, ridge directions are extracted locally. In this case, “locally” means that ridge directions are determined by only using gray scales of pixel groups near the watched pixel, so consistency with the ridge directions of the whole fingerprints is not considered. In such a method of extracting ridge directions, ridge directions are extracted locally by using gray-scale contrasts only in nearby pixel groups, so erroneous ridge directions have been extracted easily due to wrinkles or scars. Further, in areas near core and delta where ridge curvature is high, there has often been a case where accurate ridge directions cannot be extracted.
In order to correct ridge directions extracted erroneously, some methods of smoothing ridge direction patterns have been proposed. Japanese Patent Laid-Open Publication No. 5-181956 or in its U.S. Pat. No. 5,608,811 (Patent Document 4) discloses an example.
However, smoothing processing of ridge directions depends on ridge directions extracted locally in ridge direction extraction processing and the direction confidence data thereof, so even in the ridge directions to which smoothing has been performed, the accuracy thereof has limitations. For example, if erroneous ridge directions are extracted in a wide area affected by noises such as wrinkles, it is impossible to correct them by smoothing processing. Further, in areas near core and delta where ridge direction changes significantly due to high ridge curvature, it has been known that smoothing processing has no effect. In this specification, an area in which direction is stable in a relatively wide area is defined as a direction stable area, and an area in which direction changes significantly such as an area near core or delta is defined as a direction unstable area.
In order to solve this problem, Patent Document 3 discloses a method of extracting ridge directions in which consistency with the all ridge directions is improved by evaluating continuity of ridges.
However, the method described in Patent Document 3 involves a problem that it is not effective in direction unstable areas, although effective in direction stable areas.
In order to explain this example, FIG. 16B shows a case in which ridge directions are extracted by using the conventional art disclosed in Patent Document 3 with respect to the fingerprint image of FIG. 16A, which is shown by being superposed on a gray-scale image. In a direction unstable area above the core in FIG. 16A, an erroneous direction is extracted as shown by the reference numeral 51.
FIGS. 17A and 17B show processes to determine directions relating to the area shown by the reference numeral 51. In FIGS. 17A and 17B, hatched zones (e.g., F6 and F7) are high-confidence zones, and zones in which directions are shown in white backgrounds (not hatched)(e.g., E6 and E7 in FIG. 17A) are zones in which directions are determined in continuity evaluation.
In this conventional art, direction of a zone is determined by using continuity only with directions of neighboring zones. Therefore, in E6 and E7 in FIG. 17A for example, erroneous directions are determined since F6 and F7 are largely affected. Further, in the next step, since directions determined erroneously such as E6 and E7 are referred to, erroneously extracted zones will increase. As an example thereof, zones in which directions are extracted erroneously such as zones D5, D6 and D7 shown in FIG. 17B are caused.